The present invention relates to cellular telephone systems. More specifically, the present intention relates to a system for increasing the reliability of the cellular telephone system in environments having substantial multipath propagation or under conditions wherein a large number of mobile telephone units simultaneously attempt to access a base station.
Many communications systems have multiples transmitters that need to randomly access one or more receivers. A local area network (LAN) is one example of such a multiaccess system. A cellular telephone system is another. In any such system, when several transmitters attempt to transmit simultaneously, the messages may interfere or xe2x80x9ccollidexe2x80x9d with one another. A receiver cannot distinguish among the messages involved in the collision.
Two such multiaccess protocols, commonly called the xe2x80x9cAlohaxe2x80x9d and xe2x80x9cSlotted Alohaxe2x80x9d protocols, are described in Bertsekas et al., Data Networks chapter 4, Prentice-Hall, Englewood Cliffs, 1987. In the Aloha protocol, each transmitter may transmit a message at any time. Upon discovering that the transmitted message has collided, the transmitter waits a random delay time and retransmits the message. In Slotted Aloha, all messages fit into a time slot of a predetermined length. Upon discovering that the transmitted message has collided, the transmitter delays a random number of slots and then retransmits the message. In both methods, a random delay is introduced to prevent transmitters from retransmitting simultaneously.
The use of code division multiple access (CDMA) modulation is one of several techniques for facilitating communications in which a large number of system users are present. The use of CDMA techniques in a cellular telephone system is disclosed in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMethod and Apparatus for Controlling Transmission Power in a CDMA Cellular Telephone Systemxe2x80x9d and in U.S. patent application Ser. No. 07/543,496 entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System,xe2x80x9d now U.S. Pat. No. 5,103,459, both assigned to the assignee of the present invention and incorporated harein by reference.
In the above-mentioned patent, a multiple access technique is disclosed where a large number of mobile stations, each having a transceiver, communicate through base stations, also known as cell-sites, using CDMA spread spectrum communication signals. The base stations are connected to a mobile telephone switching office (MTSO), which in turn is connected to the public switched telephone network (PSTN).
The use of CDMA spread-spectrum techniques maximizes the number of mobile stations that can communicate simultaneously with the base station because the same frequency band is common to all stations. Each mobile has a pseudonoise (PN) code uniquely associated with it that the mobile station uses to spread its transmitted signal. In the above-referenced patent, this PN code is called the xe2x80x9clong PN code.xe2x80x9d Once the call has been initiated, i.e., the base station has selected the long PN code corresponding to the transmitting mobile station, the base station can receive and de-spread the signal transmitted by the mobile station. Similarly, the mobile station can receive and de-spread the signal transmitted by the base station. In some systems, the signals may be modulated with a xe2x80x9cpilotxe2x80x9d PN code as well.
However, for certain types of transmissions, it is advantageous to use a common PN long code, rather than a unique long code for each mobile station. The message transmitted by a mobile station attempting to initiate a call is one example of such a transmission. A mobile station wishing to initiate calls can transmit such requests on a common xe2x80x9caccess channelxe2x80x9d using a corresponding common PN code. The base station can monitor the access channel by despreading the signal using this PN code. The access channel is used because messages such as those for initiating a call are relatively short in comparison to voice transmissions, and a receiver could more easily monitor a relatively few access channels than the large number of unique xe2x80x9ctraffic channelsxe2x80x9d with which the mobile stations are associated by their unique PN long codes.
The access channel may be used by the mobile station not only to initiate a call, but to transmit any information to the base station at a time other than during a call that has already been initiated. For example the access channel may be used by the mobile station to respond to an incoming call initiated by a base station over a xe2x80x9cpaging channel.xe2x80x9d
Under any of the conditions discussed above, multiple mobile stations may transmit simultaneously on the access channel. When two mobile stations transmit simultaneously and there is no multipath, the transmissions arrive at the base station separated in time by a delay equal to the difference of twice the distance between each mobile station and the base station. Under most operating conditions, it is unlikely that a large number of mobile stations will be at precisely equal distances from the base stations. However, simultaneously transmitted messages would collide if two or more stations are at the same range. Under most conditions, the base station can distinguish among the transmissions because the time between arrivals of the transmissions at the base station exceeds one PN chip.
Some operating conditions tend to produce collisions. Collisions are likely to occur when a large number of mobile stations approach the edge of a cell simultaneously, a condition causing handoffs of the mobile stations. The access channel transmissions arrive at the base station simultaneously because the mobile stations are at substantially the same distance from the base station when at the edge of the cell.
It is also possible that a large number of mobile users would attempt to simultaneously initiate calls for other reasons such as following a natural disaster. The simultaneous transmissions of multiple mobile stations on the access channel may exceed the maximum throughput of the processor in the base station.
The probability of access channel collisions increases with an increase in the number of mobile stations and with an increase in multipath reflections. Multipath compounds the problem because, while the main signals of two transmissions may be separated in time by more than one chip, multipath components of the transmissions may not be. Furthermore, as discussed in U.S. Pat. No. 5,109,390, issued Apr. 29, 1992, a base station diversity receiver may have multiple correlators that combine received multipath components to improve message quality. However, ambiguities may exist between multipath components would reduce the effectiveness of the diversity receiver. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.
The present invention reduces interference between multiple spread-spectrum transmitters operating simultaneously and improves distribution of the transmissions among the available resources of the receiver. The present invention is generally applicable to any communication system having multiple transmitters attempting uncoordinated communication with a receiver, including local area networks. In an illustrative embodiment of the present invention, the transmitters are mobile stations transmitting on an access channel and the receiver is a base station in a CDMA cellular communications network.
Each mobile station uses one or more randomization methods for its access channel transmissions. The randomizations have the effect of separating the transmissions to reduce collisions. The first randomization separates the access channel signals by adding a random time delay to each signal and the second randomization separates them by randomly changing the direct sequence spreading of each signal.
In the first randomization, called xe2x80x9cPN randomization,xe2x80x9d the mobile station time-delays its access channel transmissions by a small amount that is greater than or equal to one chip but is much less than the length of the message itself. In contrast, a non-spread-spectrum communication system using a slotted aloha protocol must, upon a collision, typically wait to receive an acknowledgement of a transmission. If a collision occurred, typically detected by not receiving an acknowledgement, the mobile station must wait a random delay, typically several slots before retransmitting the message. Because the present invention addresses spread-spectrum systems, collisions are naturally reduced by the range difference described above and even more by adding the PN random delay which is typically much less than a slot length.
Although true randomization would be ideal, a pseudorandom method is used so that the base station can obtain the value of the delay used by the mobile station, which it requires to demodulate the transmission. The PN randomization delay may be pseudorandomly produced using a hash algorithm to which a number uniquely associated with that mobile station is provided. The input number may be the station""s electronic serial number (ESN). A further advantage of a pseudorandom method for calculating the PN randomization delay is that the base station, knowing the amount of delay added by a mobile station, may more quickly acquire a signal that the mobile station subsequently transmits on a traffic channel.
PN randomization may be understood in the context of a scenario involving a number of mobile stations simultaneously transmitting at the edge of a cell, i.e., equally distant from the base station. In such a scenario, PN randomization increases the effective distance from each mobile station to the base station by a random amount.
Multipath significantly increases the difficulty experienced by a base station in distinguishing the signals simultaneously transmitted by different mobile stations. The small PN randomization delay may not be enough to separate the multipath components, which would otherwise be used by a base station diversity receiver to improve reception in multipath environments.
A second randomization, called xe2x80x9cchannel randomization,xe2x80x9d may be used to improve transmission quality in such a multipath environment. As discussed in the above-referenced patents and copending application, the CDMA transmitter spreads its signal using a PN code and the CDMA receiver demodulates the received signal using a local replica of the PN code. In channel randomization, the mobile station randomly changes the PN code with which it spreads the access channel signal. Changing the PN code effectively creates a larger number of access channels. The base station has a receiver that corresponds to each possible access channel. Even in the presence of multipath, the base station can distinguish simultaneous transmissions on different access channels.
When channel randomization is used, the base station may send the mobile station a parameter representing the maximum number of access channels, i.e., the maximum number of different PN codes, that it can receive. The base station transmits this maximum access channel parameter to the mobile station during periodic communications of system information or xe2x80x9coverheadxe2x80x9d between the base station and a mobile station.
A base station may not be able to distinguish among simultaneous transmissions if it receives more such transmissions than it has access channels. For that reason, mobile stations may use a third randomization called xe2x80x9cbackoff randomizationxe2x80x9d and a fourth randomization called xe2x80x9cpersistencexe2x80x9d in addition to PN randomization and channel randomization.
Each transmission on an access channel by a mobile station attempting to communicate with a base station is called a xe2x80x9cprobe.xe2x80x9d If the base station successfully distinguishes and receives the probe, it transmits an acknowledgement to the mobile station. If the mobile station does not receive an acknowledgement to its probe after a predetermined timeout period, it attempts another probe. A predetermined number of such probes is called an xe2x80x9caccess probe sequence.xe2x80x9d The entire access probe sequence may be repeated multiple times if the mobile station does not receive an acknowledgement of any probe in the sequence.
In backoff randomization, the mobile station inserts a random delay between successive probes. Before beginning a probe, the mobile station generates a random number in a predetermined range and delays the probe by an amount proportional to the random number.
In persistence, the mobile station inserts a random delay before each access probe sequence. Before beginning an access probe sequence, the mobile station compares a randomly generated number to a predetermined persistence parameter. The persistence parameter is a probability that is used to determine whether an access probe sequence will or will not occur. The mobile station begins the access probe sequence only if the random number is within a range of numbers determined by the persistence parameter. If persistence is used, the mobile station performs the test at predetermined intervals until the test passes or until a probe is acknowledged.
Finally, if the mobile station does not receive an acknowledgment to any probes within a predetermined number of access probe sequences, it may abandon the attempt.
In a cellular telephone system, a mobile station uses the access channels for any non-voice transmissions to the base station. The mobile station may, for example, request communication with the base station when the mobile user initiates a call. The mobile station may also respond on the access channel to a transmission from the base station to acknowledge an incoming call. In the latter situation, the base station can schedule its transmissions on the paging channel to more efficiently handle the responses from the mobile stations, which may be expected to occur within a certain time period. Because the base station has some control over the situation, the mobile stations are not required to use persistence for transmitting responses.
Mobile stations may further reduce interference with each other by transmitting with the minimum power necessary for their signals to be received by the base station. A mobile station transmits its first probe at a power level somewhat less than it estimates to be necessary to reach the base station. This conservative estimate may be a predetermined value or it may be calculated in response to the measured power level of a signal that the mobile station has or is receiving from the base station. A preferred embodiment is for the mobile station to measure the received power from the base station. This received power is the transmitted power of the base station times the path loss. The mobile station then uses this estimate, plus a constant correction, plus adjustment factors to set the initial transmit power. These adjustment factors may be sent to the mobile station from the base station. Some of these factors correspond to radiated power of the base station. Since the path loss from the mobile station to the base station is essentially the same as from the base station to the mobile station, the signal received at the base station should be at the correct level, assuming that the base station has supplied the appropriate correction factors. After transmitting the first access probe at this minimum power level, the mobile station increases the power of successive probes within each access probe sequence by a predetermined step amount.
The foregoing, together with other features and advantages of the present invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.